Rust inhibitive lead pigments



Patented May 20, 1952 RUST INHIBITIVE LEAD PIGMENTS Leonard M. Kebrich,Brooklyn, N. Y., assignor to National Lead Company, New York, N. Y., acorporation of New Jersey No Drawing. Application July 3, 1947, SerialNo. 759,001

4 Claims.

This invention relates to red lead pigments and particularly tocompositions of red lead having superior [pigment properties.

Red lead has long been considered a desirable pigment for use in paintsfor protection of ferrous articles. This pigment, as heretoforemanufactured and known has consisted of predominantly true red lead,PbsO4, and has inevitably contained some percentage of free, uncombinedlead monoxide, PbO. The percentage of free litharge in a pure grade ofred lead is low, often of the order of a few per cent; however, this PbOcontent has been responsible for some undesirable characteristics. Thus,the PbO' is often reactive with paint vehicles causing storagedifficulties and in addition, when the paint is exposed to the weather,the PhD tends to form lead carbonate resulting in early loss of gloss,formation of a white surface film which destroys the original color, andwhich also may initiate premature breakdown of the film. The prior arthas therefore attempted to provide superior red lead pigments byproviding as high a true red lead content as possible, e. g. containing97% or even slightly higher Pb3O4 content. Such a high Pb304 content isobtained by prolonged furnacing of PbO, and economic factors mitigateagainst production of much purer grades.

It is therefore, the principal object of this invention to provide a redlead composition containing substantially no uncombined or free lithargePbO. It is a further object to provide a red lead composition havingsuperior pigment properties. Another object of the invention is toprovide red lead composition suitable for use as a pigment whichcontains a lower Pb304 content than has heretofore been considereddesirable. It is still another object to provide a novel and economicalmethod for manufacturing the unique red lead compositions of thisinvention.

This invention in its broadest aspect contemplates the production of alitharge free red lead composition by treating red lead, as commerciallymanufactured, with chromic acid, so that the free litharge originallypresent in the red lead is converted to lead chromate. This isaccomplished by making a suspension, or slurry of the red lead withwater and slowly adding thereto with agitation, a solution of chromicacid. Under these conditions, the litharge in the red lead may beconverted to either normal or basic lead chromate depending on theproportionate amounts of chromic acid added and the amount of freelitharge present. If it is desired to obtain a faster reaction, a smallamount of a solutionforming catalyst such as acetic 0r nitric acid maybe added; however, adequate reaction speed may be obtained even atnormal room temperature Without its use. If no acid catalyst is used,the product may be simply filtered and dried, without washing, as iswell known to the art.

It is important that the chromic acid solution be added slowly so thatno large excess of chromate ions is at any time present in the reactingsolution. The reaction temperature is not critical, normal roomtemperature gives good results, however, higher temperatures, up toabout C. may be used to accelerate the'reaction speed. Under theseconditions, it will be found that the free litharge alone combines toform lead chromate, the red lead itself being unaffected. Also,substantially all the free litharge in the red lead can be so converted,resulting in a substantially litharge free product.

The following examples will serve to illustrate the preparation of thepure chromated red lead products of this invention.

EXAMPLE I Monobasic chromated red lead 2,400 grams of red leadcontaining 76.36% true red lead were agitated with 12 liters of water atroom temperature in a suitable vessel. 127.1 grams of chromic aciddissolved in 1.1 liters of water-were slowly dropped into the suspensionover a 5 hour period. The product was filtered and dried at 80 C. Thedried product had a deep orange color and on analysis was found tocontain 71.82% red lead Pb304, and 28.18% monobasic lead chromate,PbO.PbCrO4. An aqueous extract of this product had a pH value of 7.0 anda microscopic examination of the product revealed that the basic leadchromate was in an extremely fine condition, and intimately dispersedthroughout the larger red lead particles. The specific gravity of thisproduct was found to be 8.21. I

EXAMPLE 11 Normal chromated red lead either basic or normal leadchromate. For use as' a rust inhibitive'pigment the basic chromatecompounds are generally ipreferred; however, the

normal lead chromate has a distinctive yellow: color which may be moredesirable for. certain.

applications. Also, an amount of chromic acid which will result in amixtureiof normal andabasic lead chromates may be used iftdesir'eds Bystarting with commercial red lead, it is possible to produce by themethodof.-this.:inyen== tion a rust inhibitive pigmentsubstantially-free:

from uncombined litharge; this pigment comprising a red lead compositioncontaining 'lead chromate. The lead chromate portion of this mixture hasof 1 itself valuable rust i inhibitive properties and its incorporationinto the red lead" composition enhances the properties *of the originalred lead. Thesame effect'cannotbe obtained 1 "by; simply: mixing redlead and lead chromate, for "under these 1 conditions, the freelithargezalready present in :the red lead' will not havebeeneliminated.'Itis' also apparent that lower-grades of redilead, that isthose'containing a substantial portion of uncombinedzleadmonoxide'xwhich are much" more economical to produce in view? of the?fact that? they are considerably easier-to manufacture, may be used as.the raw material fortheproduction'of this invention, and

whileias low as 50 per cent'PbsO r red lead may easily be-convertedxintovaluable rustinhibitive pigments. A 75% .-or 85%'red lead issufiiciently economical to produce. and-may be preferred for generaluse.

' Chromated :red' :leads according t"this: inventionmay: bemade'intotpaint in" any manner' well known to the art. To'illustrate'thez-superior properties of paint.- made. from the product ofsimultaneously to actual weatheringconditions.

Such exposure tests take-a long-time to finally evaluatepbut afte1x136days of exposure, both the. red lead anduthebasic" lead chromate.-paintsshowed the-vpresence of a carbonaterefilm while theichromatedfred lead:paint showed no "change- The initial formation in color. or appearance:of the carbonate film on the red lead and basic lead chromatepaintsswould tend to indicate a definitely shorter total life than fromthe chromated red lead paint which apparently was thus far-unaffected.

sThe-zohromatedmed lead pigment according to thisuinvention has:numerous important advantages over conventional red lead. A lower gradered lead may be used in this invention in the production of a high graderust inhibitive paint at considerable manufacturing economy. The keepingqualities of paints made with the product of this invention are good andthe paints may. lee-kept 'instorage forsa long :period withoutdeterioration and hardening in thecontainer. Chromated lead as hereindescribed is compatible with substantially all common paint ve-..hic1es,..,and..with. many natural and synthetic resins. "Due to'thepresence of lead chromate in -;.theired;.lea'd composition, the rustinhibitive .-proper.ties.:arez-distinctly enhanced. The glossretention-and 'colorfastness of paints made with .the Chromated red leadof this invention as the :pigment phase are definitely superior toeither red lead or basic lead chromate when used alone.

The-use-of the product of this invention eliminates substantially allthe disadvantages resultingirom the presence of free, uncombined leadmonoxide,-Pb0, in red lead as previously used.

It is to be understood by those skilled in the artthat variousproportions of'red leadand lead a chromate may beobtained by varying thegrade of'red leadused'and the amount of'chromic acidor chromate salt:added thereto. This invention 'is 'not limited to any definiteproportion of red lead-and lead chromate; and,--manufacturing economicsmay-dictate the most desirable proportion for commercialuse.

I claim:

- 1. A- process formanufacturing a lithargeiree red-lead.composition-which comprises forming. a water slurry of red: leadcontaining free PbO and slowly adding to saidslurry chromic acid 'in'amount to' convert said' free' PbO to leadichromate: and filteringanddrying the product.

2. A process for manufacturing a'litharge free red leadcomposition'which-comprises forminga water slurry of red lead"containing free PbO andzslowlyadding-to :said. slurry chromic acidinlamount-to convert said free PbO' to normal lead chromate.andifiltering. and drying the product.

3. Aprocessfortmanufacturing a litharge free red lead compositionwhichcomprises forming a water slurry oft'red'lead containing free P130and slowly adding to said slurry chromic acidin -amounttorconvert'said'free PbO to basic lead chromate and: filtering anddrying the product.

fi'Aprocess for manufacturinga litharge free red lead" composition whichcomprises forming a water slurry'of red lead containing a substantialportion' of free P and slowly adding to saidislurrysch'romic acid innamount to-convert said'freePbOtolead chromateand filtering and drying:the:product.

LEONARD M. KEBRICH.

.REFERENGES. CITED The following references are of record in the file01- this patent:

I UNITED STATES. PATENTS Number Name j Date 987,965 Cushman Mar. 28;1911 2,044,244 Harshaw' June 16, 1936

1. A PROCESS FOR MANUFACTURING A LITHARGE FREE RED LEAD COMPOSITIONWHICH COMPRISES FORMING A WATER SLURRY OF RED LEAD CONTAINING FREE PHOAND SLOWLY ADDING TO SAID SLURRY CHROMIE ACID IN AMOUNT TO CONVERT SAIDFREE PBO TO LEAD CHROMATE AND FILTERING AND DRYING THE PRODUCT.